Kaggle Optimal Blending: The 80/20 Rule

Problem

Re-ranking (k-reciprocal, Jaccard, etc.) is powerful but using 100% re-ranked results can lose valuable signal diversity. Equal-weight blending often underperforms.

Context / Trigger Conditions

Use when:

• Implementing post-processing for Re-ID, retrieval, or recommendation tasks
• Re-ranking alone doesn't give expected boost
• Ensemble of multiple models shows little improvement
• Kaggle score plateaus despite strong individual models

Solution

The 80/20 Optimal Blending Rule

Key insight from 0.938 mAP solution (Jaguar Re-ID):

blend_raw_ratio = 0.20  # 20% raw, 80% re-ranked

final_similarity = (
    (1 - blend_raw_ratio) * rerank_score +
    blend_raw_ratio * raw_score
)

Why 80/20?

• 80% re-ranking: Captures neighborhood structure, removes false positives
• 20% raw: Preserves genuine long-range similarities, maintains diversity
• This ratio emerged as optimal through experimentation (0.938 mAP)

Implementation

import numpy as np

def optimal_blend(similarity_matrix, reranked_matrix, raw_ratio=0.20):
    """
    Blend re-ranked and raw similarity matrices

    Args:
        similarity_matrix: Original NxM similarity matrix
        reranked_matrix: Re-ranked version (k-reciprocal, etc.)
        raw_ratio: Weight for raw score (default 0.20)

    Returns:
        Blended similarity matrix
    """
    return (1 - raw_ratio) * reranked_matrix + raw_ratio * similarity_matrix

Re-Ranking Parameters (Optimized for 0.938 mAP)

# k-Reciprocal Re-ranking parameters
k1 = 20   # First k for reciprocal neighbors
k2 = 6    # Second k for final re-ranking
lambda_param = 0.2  # Jaccard weight

Complete Post-Processing Pipeline

def post_process_for_reid(query_features, gallery_features):
    """
    Complete post-processing: TTA + QE + Re-ranking + Optimal Blend
    """
    # 1. TTA (Test Time Augmentation)
    # query_features = (f_original + f_flipped) / 2

    # 2. Query Expansion (optional)
    # Expand with top-k gallery samples
    qe_top_k = 3

    # 3. Compute raw similarities
    raw_sim = cosine_similarity(query_features, gallery_features)

    # 4. Re-ranking (k-reciprocal)
    reranked_sim = k_reciprocal_reranking(
        raw_sim,
        k1=k1,
        k2=k2,
        lambda_param=lambda_param
    )

    # 5. Optimal blending (THE KEY!)
    final_sim = optimal_blend(
        raw_sim,
        reranked_sim,
        raw_ratio=0.20  # Don't use 0.0 or 0.5!
    )

    return final_sim

Verification

Test the blend ratio:

# Test different ratios
for ratio in [0.0, 0.10, 0.15, 0.20, 0.25, 0.30]:
    blended = optimal_blend(raw_sim, reranked_sim, raw_ratio=ratio)
    score = evaluate(blended, val_labels)
    print(f"Ratio {ratio}: {score:.4f}")

Expected pattern:

• 0.0 (100% re-rank): Good but may over-prune
• 0.10: Better
• 0.20: Optimal ← Use this!
• 0.25: Slight decline
• 0.50: Significant decline (too much raw)

Example

Jaguar Re-ID Competition:

# Without optimal blending
# Re-ranking only: 0.92 mAP
# Raw only: 0.85 mAP

# With optimal blending (80/20)
final_score = 0.8 * reranked + 0.2 * raw
# Result: 0.938 mAP (+1.8% over re-ranking alone)

H-Blend Strategy (0.944 mAP solution):

# Single strong model should dominate
weights = {
    'model_0.944': 0.95,  # Dominant!
    'model_0.938': 0.02,
    'model_0.937': 0.02,
    'model_0.930': 0.01,
}

This validates the "ensemble-model-correlation-trap" skill.

Notes

When to Use 80/20

✅ Person/Animal Re-ID tasks ✅ Image retrieval with re-ranking ✅ Recommendation systems with neighborhood methods ✅ Any metric learning task with post-processing

When to Adjust the Ratio

• Very clean data: Try 0.15 (more re-ranking)
• Noisy data: Try 0.25-0.30 (more raw signal)
• High threshold requirements: Try 0.10

Common Mistakes

❌ Using 100% re-ranking (loses diversity) ❌ Using 50/50 blend (loses re-ranking benefit) ❌ Equal-weight ensembles (use 95/5 for strong/weak) ❌ Tuning ratio on test set (use validation only!)

Related Skills

• ensemble-model-correlation-trap: Why strong models should dominate
• k-reciprocal-reranking: Implementation details
• eva02-reid-backbone: Produces strong features for blending

References

• Kaggle Jaguar Re-ID: sanidhyavijay24 (0.938 mAP)
• Kaggle Jaguar Re-ID: nina2025 H-Blend (0.944 mAP)
• k-Reciprocal Re-ranking: "Re-ranking Person Re-identification" (CVPR 2017)